Collagen is a fundamental protein found in connective tissue in animals, and it is present in the cornea and sclera of the eye. Several eye disorders are related to defects in collagen structure and include keratoconus, keratectasia, progressive myopia, and possibly glaucoma.
Keratoconus is a debilitating, progressive eye disorder, which is believed to occur due to progressive slippage of collagen lamellae in the cornea, usually bilateral, beginning between ages 10 and 20. The cornea develops a conical shape, causing significant changes in the refractive power of the eye. While corrective lenses may help vision, corneal transplant surgery may be necessary if eyeglasses or contact lenses are inadequate. THE MERCK MANUAL OF DIAGNOSIS AND THERAPY 722 (Mark H. Beers and Robert Berkow eds., 17th ed. 1999).
Keratoconus is estimated to affect 1 person in about 435 to 2000 people in the general population. In its classical form, keratoconus commences at puberty and progresses into the third to fourth decade of life Rabinowitz, Y. S. “Keratoconus,” Surv. Opthal. 1998; 43(4):297-319. Thus, its overall impact is magnified by virtue of the younger population that it afflicts. Clinically, the disease is marked by progressive thinning of the corneal stroma with resultant bulging and distortion of the thinned, weakened areas. This thinning and distortion is documented by optical and ultrasonic methods. The bulging, distorted cornea creates an optically imperfect surface to the eye that produces an increasingly irregular astigmatism and myopia. Contact lenses are used to correct these optical imperfections when spectacle lenses are no longer able to compensate for the induced optical distortion. When contact lens correction fails, only a corneal transplant will allow restoration of visual function. The need for corneal transplantation arises when the disease has progressed and central corneal scar formation occurs, or the distortion is so great that contact lenses can no longer be worn.
Although the underlying etiology of keratoconus remains unclear, there are two main mechanistic theories currently entertained. The first is related to destabilization of collagen lamellae through increased degradation via imbalances in endogenous proteases and/or their inhibitors. In this regard, the scientific evidence has been somewhat equivocal with some studies showing increased matrix-metalloproteinase activity and others reporting no change (reviewed by Collier, S. A., “Is the corneal degradation in keratoconus caused by matrix-metalloproteinases?” Clin. Exp. Ophthalmol. 2001; 29:340-344). An alternative theory regards collagen fibril slippage with no overall tissue loss. Meek, K. M., et al. have shown, using synchrotron X-ray scattering, that stromal lamellar organization is altered with an associated uneven distribution of collagen fibrillar mass. These changes are consistent with inter- and/or intra-lamellar slippage within the stromal layers of the keratoconic cornea, leading to central thinning. Meek, K. M., et al., “Changes in collagen orientation and distribution in keratoconus corneas,” IOVS 2005; 46(6):1948-1956. The defect that would allow such slippage could be related to changes in the collagen to proteoglycan interactions and/or qualitative changes in the fibrillar collagens. Regarding this second point, very little is known about the qualitative biochemical collagen changes that occur in keratoconus. However, alterations in difunctional collagen cross-linking were reported decades ago. Cannon, J. and Foster, C. S., “Collagen crosslinking in keratoconus,” IOVS 1978; 17(1):63-65; Oxlund, H. and Simonsen, A. H., “Biochemical studies of normal and keratoconus corneas,” 1985; 63:666-669; Critchfield, J. W., et al., “Keratoconus: I. biochemical studies,” Exp. Eye Res. 1988; 46:953-963. Regardless of the exact mechanism responsible for progressive corneal thinning, the pathologic changes that take place are accompanied by a loss of biomechanical strength. In this regard it has been shown that keratoconic corneas show a decreased stress for a given strain as compared to controls (i.e., decreased tissue stiffness) [Andreassen, T. T., et al., “Biomechanical properties of keratoconus and normal corneas,” Exp. Eye Res. 1980; 31:435-441.] Andreassen, T. T., et al. also found that keratoconus collagen displayed a decreased resistance to enzymatic digestion with pepsin, a finding which is consistent with alterations in collagen cross-linking.
Current treatments for keratoconus either mask the surface irregularity with a variety of contact lenses, or attempt to improve the surface contour with intracorneal ring segments, lamellar keratoplasty, or excimer laser surgery. Binder, P. S., et al., “Keratoconus and corneal ectasia after LASIK,” J. Refract. Surg. 2005; 21:749-752. However, the disease is progressive and none of these options obviates the need for eventual corneal transplantation.
Glaucoma is a group of disorders characterized by progressive damage to the eye at least partly due to increased intraocular pressure, the aqueous pressure in the eye. Increased intraocular pressure results from an inadequate aqueous outflow from the eye due to an obstruction in the trabecular meshwork from which the eye drains. Collagen is necessary to maintain the structural integrity of the trabecular meshwork. Rehnberg, M., et al., “Collagen distribution in the lamina cribosa and the trabecular meshwork of the human eye.” Brit. J. Ophthalmol. 71:886-92 (1987). Open-angle glaucoma can be treated with medical, laser, or surgical therapy to prevent damage to the optic nerve and visual field by stabilizing the intraocular pressure. THE MERCK MANUAL OF DIAGNOSIS AND THERAPY 733-36 (Mark H. Beers and Robert Berkow eds., 17th ed. 1999).
In myopia, or nearsightedness, the image of a distant object is focused in front of the retina because the axis of the eyeball is too long or the refractory power of the eye is too strong. Rays of light fall in front of the retina because the cornea is too steep or the axial length of the eye is too long. Without glasses, distant images are blurry, but near objects can be seen clearly. While glasses or contact lenses correct vision, refractive surgery decreases a patient's dependence on glasses or contact lenses. Progressive myopia is a condition associated with high refractive error and subnormal visual acuity after correction. This form of myopia gets progressively worse over time. THE MERCK MANUAL OF DIAGNOSIS AND THERAPY 741-43 (Mark H. Beers and Robert Berkow eds., 17th ed. 1999). The development of severe myopia is associated with scleral thinning and changes in the diameter of scleral collagen fibrils in humans. McBrien, N. A., et al., “Structural and Ultrastructural Changes to the Sclera in a Mammalian Model of High Myopia.” Investigative Ophthelmol. & Visual Sci. 42:2179-87 (2001).
Refractive surgery alters the curvature of the cornea to allow light rays to come to focus closer to the retina, thus improving uncorrected vision. In myopia, the central corneal curvature is flattened. However, ideal candidates for refractive surgery are people with healthy eyes who are not satisfied wearing glasses or contact lenses for their daily or recreational activities. Candidates for refractive surgery should not have a history of collagen vascular disease because of potential problems with wound healing. As keratoconus is a progressive thinning of the cornea, thinning the cornea further with refractive surgery may contribute to the advancement of the disease. Huang, X., et al., “Research of corneal ectasia following laser in-situ keratomileusis in rabbits.” Yan Ke Xue Bao, 18(2):119-22 (2002), The side effects of refractive surgery include temporary foreign-body sensation, glare, and halos. Potential complications include over- and under-correction, infection, irregular astigmatism, and, in excimer laser procedures, haze formation. Permanent changes in the central cornea caused by infection, irregular astigmatism, or haze formation could result in a loss of best corrected acuity.
Keratectasia is the protrusion of a thinned, scarred cornea. In laser in situ keratomileusis (LASIK), if the laser removes too much tissue, or the flap is made too deep, the cornea can become weak and distorted, leading to keratectasia. LASIK is contraindicated for patients with thin corneas, or those with keratectasia as a result of a prior LASIK procedure. Rigid gas permeable contact lenses are the recommended treatment for correcting vision in these patients. Kim, H., et al., “Keratectasia after Laser in situ Keratomileusis.” Int'l, J. Ophthalmol. 220:58-64 (2006).
A major breakthrough in the treatment of keratoconus and related keratectasias has been realized. Recent work by the German group of Wollensak, Spoerl, and Seiler has shown that cross-linking corneal collagen through application of riboflavin and ultraviolet light (UVR) can limit progressive vision loss in keratoconus patients. This modality represents a method through which stabilization of the corneal collagen lamellae and has been shown to prevent the progressive thinning of the cornea and loss of vision observed in keratoconus patients. This treatment involves the serial applications of riboflavin (0.1%) onto a de-epithelialized human cornea followed by exposure of the riboflavin saturated tissue to ultraviolet radiation in a UVA-370 nanometer wavelength region, at 3 mW/cm2 radiant energy. The patient is treated with antibiotic drops to prevent infection and oral pain medicine after the procedure. Literature accruing over the past 9 years has described the utility of photochemical cross-linking using UVA irradiation (2λmax=370 nm) with riboflavin as a photosensitizer (UVR). The work of the German group of Wollensak, G., Spoerl, E., and Seiler T., has shown that this method of cross-linking the collagen within the corneal stroma has proven effective in limiting the progression of corneal thinning, distortion, and resulting optical degradation of the eye. Wollensak, G., et al., “Riboflavin/ultraviolet-A-induced collagen crosslinking for the treatment of keratoconus.” Am. J. Ophthalmol. 2003; 135:620-27. Despite these successes, the UVR therapy poses attendant risks, particularly related to ultraviolet irradiation. As such, this therapy has yet to gain FDA approval in the US.
Because riboflavin tissue penetration is limited by the corneal epithelium, it is necessary to remove the corneal epithelium by scraping prior to riboflavin application. Removal of the corneal epithelium exposes the cornea to a risk of infection and causes significant pain. In addition, keratocyte (Wollensak, G., et al., “T. keratocyte cytotoxicity of riboflavin/UVA treatment in vitro.” Eye, 18:718-22 (2004); Wollensak, G., et al., “Keratocyte apoptosis after corneal collagen cross-linking using riboflavin/UVA treatment.” Cornea, 23(1):43-49 (2004)) and corneal endothelial cell toxicity (Wollensak, G., et al., “Corneal endothelial cytotoxicity of riboflavin/UVA treatment in vitro.” Ophthalmic Res., 35:324-28 (2003)) can occur with application of riboflavin/UVA to the cornea. In a similar manner, application of this therapy to the posterior sclera has been reported to damage cells in the photoreceptor, outer nuclear, and retinal pigment epithelial layers (Wollensak, G., et al., “Cross-linking of scleral collagen in the rabbit using riboflavin and UVA,” Acta Ophthalmologica Scandinavica, 83:477-82 (2005).
Currently, clinical trials are ongoing in Europe (Caporossi, A., et al., “Parasurgical therapy for keratoconus by riboflavin-ultraviolet type A rays induced cross-linking of corneal collagen: Preliminary refractive results in an Italian study,” J. Cataract Refract. Surg. 2006; 32:837-845; Wollensak, G., “Crosslinking treatment of progressive keratoconus: new hope,” Cur. Opin. Ophthal. 2006; 17:356-360) with significant interest generated for initiating clinical trials in the United States. The early reports from this therapy are encouraging. After 5 years in the Dresden study, individuals who have undergone this treatment protocol have yet to show progression of their keratoconus. With these encouraging results, corneal cross-linking therapy is being extended to include patients with related disorders such as the ectasia that occurs following LASIK (Laser-Assisted In situ Keratomileusis) and PRK (Photorefractive Keratectomy) excimer refractive surgery (Binder, P. S., et al., 2005). These are devastating complications of keratorefractive surgery in today's clinical practice. Anecdotal reports have also emerged reporting the use of collagen cross-linking as an effective means to control difficult-to-treat corneal fungal infections and corneal melts.
Despite these successes, the UVR therapy poses attendant risks, particularly related to ultraviolet irradiation. As such, this therapy has encountered difficulty gaining FDA approval and is currently unavailable in the United States. Because free oxygen radical formation occurs with riboflavin photolysis (Baier, J., et al., “Singlet oxygen generation by UVA light exposure of endogenous photosensitizers,” Biophys. J. 2006; 91:1452-1459), this cross-linking method has a negative impact on cell viability. Indeed, keratocyte (Wollensak, G., et al., 2004) and corneal endothelial cell toxicity (Wollensak, G., et al., 2003) does occur with application of this therapy to the cornea. As a result of such toxicity, it has been recommended that patients with particularly thin central corneas (<40 μm) not undergo this therapy since the depth of UVA penetration exposes the endothelial cells (which are vital to maintaining corneal clarity through water regulation) to toxic photochemical damage. Furthermore, the long-term risks of this photochemical exposure are not known. Secondly, deep tissue penetration by the riboflavin requires removal of the corneal epithelium, a procedure that increases morbidity and complications. This requires analgesics and antibiotics following the UVR cross-linking procedure.
A need in the art exists to develop a topical self-administered compound in order to produce a comparable degree of collagen cross-linking to the UVR therapy. Also needed are methods and compositions for modulating (e.g., increasing or decreasing) the rate or degree at which such compounds effectuate collagen cross-linking.